Grain refinement is an important measure to improve the metallurgical quality of cast steel, and it has been a research subject of great concern to thein various industrial countries.However, due to the high temperature of molten steel and the peritectic transition of the largest amount of carbon steel and low alloy steel during solidification process, the situation is very complicated and the research work is difficult. Although the research work in this field was started more than 60 years ago, so far, no stable process can be promoted and applied. In this regard, the space dependence for exploration, research and innovation is still lacking.But it’s very broad.
The addition of Ti or Nb to steel can refine the grain size, and its mechanism is mainly pinning rather than forming heterogeneous nuclei. Therefore, there may be many negative effects on the performance of steel. Therefore, it is not advisable to adopt it without further recognition.
From the current understanding, adding REE into final deoxidized steel liquid is a promising process measure for grain refinement of cast steel. Although a lot of research work has been done before, it is still preliminary.For steel grades (high manganese steel, fully austenitic stainless steel, carbon steel with high carbon content and low alloy steel) which do not undergo peritectic transformation during solidification, the effect of REE on grain refinement is affirmative. The key lies in the process method that various process parameters are further optimized through repeated tests to achieve good results and reproducibility.
According to the research work done by Professor Tuttle in the United States, the effect of REE on grain refinement of carbon steel and low alloy steel with the largest amount of REE is affirmative, but up to now, the effect of REE on carbon steel and low alloy steel is not stable because the mechanism of action is still insufficient.There is still a lot of testing and research to be done in this respect.
In addition, due to the strong activity of rare earth oxide at high temperature, it is easy to interact with oxide inclusion in molten steel and even refractory material of pouring bag, forming viscous compound, which leads to slag hole defects in castings and the possible failure of pouring bag blocking pouring hole.Therefore, appropriate precautions should be taken in both smelting and pouring processes.At present, the simple and feasible measure is to use high aluminium refractory in the casting ladle, in which Al2O3 should not be less than 70%.